Table Of ContentMerging clusters of galaxies observed with
XMM-Newton
E. Belsolea J-L. Sauvageotb G.W. Prattc and H. Bourdind
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H.H. Wills Physics Laboratory - University of Bristol, Tyndall Avenue, Bristol
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2 BS8 1TL, UK
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n Service d’Astrophysique, CE-Saclay, L’orme des Merisiers, 91191 Gif sur Yvette
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Cedex, France
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8 MPE Garching, Giessenbachstraße, 85748 Garching, Germany
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Dipartimento di Fisica, Universita` degli studi di Roma Tor Vergata, Via della
1 Ricerca Scientifica 1, 00133 Roma, Italy
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5 Abstract
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We present results from the XMM-Newton observations of our ongoing program
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- on merging clusters. To date three clusters have been observed, covering the tem-
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r poral sequence from early to late stage mergers: A1750, A2065 and A3921. Using
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s spatially-resolved spectroscopy of discrete regions, hardness ratio and temperature
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maps, we show that all three clusters display a complex temperature structure. In
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the case of A1750, a double cluster, we argue that the observed temperature struc-
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X ture is not only related to the ongoing merger but also to previous merger events.
r A2065 seems an excellent example of a ‘compact merger’, i.e. when the centres of
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the two clusters have just started to interact, producing a shock in the ICM. Us-
ing comparisons with numerical simulations and complementary optical data, the
highly complex temperature structure evident in A3921 is interpreted as an off-axis
merger between two unequalmass components. Theseresults illustrate the complex
physics of merger events. The relaxation time can be larger than the typical time
between merger events, so that the present day morphology of clusters depends not
only on on-going interaction but also on the more ancient formation history.
Key words: galaxies: clusters: general – galaxies: clusters: individual: Abell 1750,
Abell 2065, Abell 3921 – galaxies: clusters: intergalactic medium, mergers –
X-rays:general – X-ray:galaxies:clusters – cosmology: large-scale structure of
Universe
PACS:
Preprint submitted to Elsevier Science 2 February 2008
1 Introduction
As the largest assembled structures in the Universe, clusters of galaxies are
commonly thought to form by gradual accretion of matter along filaments
and by interaction and merging with previously formed structures. Hydrody-
namical simulations (e.g. Rowley et al. 2004 and references therein) predict
that merger events strongly affect the physical characteristics of the intra-
cluster medium (ICM). In particular the temperature structure is thought to
be an excellent indicator of the cluster dynamical state and formation his-
tory. Spectro-imaging observations with XMM-Newton and Chandra allow
the building of precise temperature maps, enabling deeper investigation of the
dynamical processes of cluster formation and the effect of the mergers on the
ICM (e.g., Markevitch et al., 2000; Neumann et al., 2003; Henry et al., 2004;
Krivonos et al., 2003; Vikhlinin & Markevitch, 2003).
With the aim of describing an evolutionary sequence of cluster formation, we
haveselectedasmallsampleofgalaxyclustersshowingmorphologicalevidence
of ongoing merger activity. The sample was selected, on the basis of previous
X-ray observations, from nearby clusters which entered the field of view of
XMM-Newton. Here we summarise results for A1750 and A3921, presented in
detail in Belsole et al. (2004, 2005), and we describe preliminary, new results
for A2065.
2 Observations and data analysis
In Table 1 we list the main physical characteristics of the three clusters, to-
gether with basic observationinformation.The A1750andA3921 observations
were very little contaminated by soft proton flares (details of the data prepa-
ration can be found in Belsole et al. 2004, 2005). Unfortunately, emission from
soft protons dominates the whole observation of A2065. This required an ad-
hoc treatment for these data, which allowed us to model the background.
A2065 will be re-observed with XMM-Newton, and thus here we discuss only
preliminary results.
The background estimates were obtained using a blank-sky observation con-
sisting of several high-latitude pointings with sources removed (Lumb et al.
(2002)), and source and background events were corrected for vignetting us-
ing the eviweight task in the Science Analysis System (SAS), enabling us
to use the on-axis response matrices and effective area.
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Table 1
Journal of observations.
Parameter Abell 1750 Abell 2065 Abell 3921
z 0.086 0.072 0.096
h m s h m s h m s
RA (J2000) 13 30 52 15 22 42.6 22 49 38.6
Dec(J2000) −01o50′27′′ +27o43′′21′′ −64o23′15′′
Obs time (ks) 30 23 30
NH(1020 cm−2)a 2.39 2.95 2.94
b
Global T (keV) 3.87 (2.84) 5.4 5.0
a
From Dickey & Lockman (1990)
b
A1750C (A1750N)
3 Results
We show in Figure 1 the images of A1750 and A3921 in the 0.3-7.0 keV band;
for A2065 a lower energy band [0.1-1.4 keV] is shown to enhance cluster emis-
sion above the strong particle background. We observe that the morphology
of the clusters is very different, reflecting already a difference in their dy-
namical state. A1750 is a prototype of a binary cluster, although the system
appears to have at least three components (Beers et al., 1991). The main clus-
ter, A1750C, is at the centre of the field of view, and a sightly less massive
cluster (A1750N) lies to the north. A2065 looks rather unperturbed on the
outskirts, but the central regions display a clear compression of the isophotes
towards the south-east, and a sharp edge to the east. Finally, the X-ray image
of A3921 is dominated by the main cluster at the centre of the field of view,
and we observe an elongation of diffuse emission to the west, where the X-ray
emission from two of the three brightest galaxies in the cluster can also be
seen.
3.1 Morphology
One approach to evaluate any deviation from a dynamically relaxed cluster is
to assume a β-model (Cavaliere & Fusco-Femiano, 1976) as a description of a
relaxed isothermal system. The large soft proton contamination of the A2065
observation reduced the significance of any substructure when this method
was applied and thus we do not discuss it further.
We thus generated low energy (0.3-2.0 keV, where the density distribution is
least temperature dependent) surface brightness maps for A1750 and A3921,
and fitted them with a bi-dimensional β-model. The two clusters belonging
3
(a) (b)
(c)
Fig. 1. (a) XMM-NewtonEPIC 0.3-7.0 keV energy band count image of A1750;
(b) MOS1+MOS2 0.1-1.4 keV energy band count image of A2065, the band
chosen to enhance cluster emission above the strong particle background; (c)
XMM-NewtonEPIC 0.3-10 keV energy band count image of A3921. These are raw,
non-background subtracted images.
to A1750 were taken into account simultaneously. We found that A1750 does
not show significant residuals above a β-model, in the region between the two
clusters, but there is some excess emission in their cores.
On the other hand, A3921 shows large residuals to the west (other than those
4
(a) (b)
Fig. 2. Surface brightness maps of A1750 (a) and A3921(b) in the soft energy band
(0.3-2.0 keV). Contours are the residuals above a 2D β-model which was subtracted
from the image.The first contour is at 1σ.
in the centre of the main cluster), indicating that a significant substructure is
present and it is related to one or both the brightest galaxies in that area. The
h m s − ◦ ′ ′′
two galaxies are at 7 arcmin (RA = 22 48 49 , Dec = 64 2310 (J2000)) to
h m s − ◦ ′ ′′
the north-west (BG2) and at 8 arcmin (RA = 22 49 04 , Dec = 64 2035
(J2000)) to the WNW (BG3) from the central brightest galaxy BG1 (RA =
h m s − ◦ ′ ′′
22 49 58 ; Dec= 64 2546 (J2000); see Belsole et al. 2005 for details).
3.2 Temperature distribution
We computed temperature maps for A1750 and A3921 by applying the multi-
scale spectro-imagery algorithm described in (Bourdin et al., 2004). EMOS
cameras only were used for this analysis. For A2065, we obtained an hardness
ratio (HR) map once the particle contamination was modelled and subtracted
from the images. This gives us qualitative, but significant results about tem-
perature substructure in this cluster. The temperature maps are shown in
Figure 3.
All three clusters show significant temperature variations, even in the cases
where surface brightness substructures were not detected (A1750). As is con-
firmed by extraction of spectra from discrete regions (Belsole et al., 2004),
the region between A1750N and A1750C has a temperature which is signif-
icantly (30%) hotter than the global temperature of either the two clusters.
Additionaltemperaturegradientsareobserved inA1750C,indicating thatthis
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(a) (b)
(c)
Fig.3.Temperaturemaps.Thewavelet algorithmdescribedinBourdin et al.(2004)
was used for A1750 (a) and A3921(c). In the case of A2065 (b) the image is an
hardness ratio map (see text for details).
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cluster was already perturbed in the past.
The temperature enhancement, coincident with the isophotal compression,
observed in the A2065 HR map points to a bow shock. We also detect a cool
core, coincident with the peak of X-ray emission.
The orientation of the striking hot temperature bar in A3921 represents our
strongest proof against the interpretation that this is a merger before close
core passage. The temperature here varies between 7 and 8 keV, up to 60%
higher than the main cluster temperature.
4 Discussion
For each cluster we summarise the results obtained in the previous sections.
A1750
(1) From the morphological analysis we found: off-centre cores and twist of
the isophotes, but lack of substructure in the region between the two
clusters.
∼
(2) thetemperatureincreases weakly ( 30%),but significantly, intheregion
between the two clusters
(3) strongtemperaturevariationsareobservedwithinthemaincluster(A1750C)
and we also measured a discontinuity in the gas density profile of order
20%.
(4) we found a high entropy in the core of the two clusters (see Belsole et al.
2004)
The combination of these results lead us to interpret A1750 as an ongoing
merger between two clusters of similar masses, which have just started to in-
teract at a real distance comparable to their virial radii. These two units will
beinthecompactphase(corepassage)within1Gyr.However thetemperature
and density variations within A1750C itself are not explicable by the merger
event occurring with A1750N and are intrinsic to this cluster. The most likely
interpretation is that this is the signature of (a) previous merger(s). A1750
is a good observational test case to be compared with numerical simulations,
since it shows all of the signatures expected from gravitationally dominated
processes. However, the fact that effects of a previous merger are observed
strongly supports the necessity of taking into account time dependent quan-
tities, such us the relaxation time, in using galaxy clusters for cosmological
purposes.
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A2065
The surface brightness shows an isophotal compression towards the south-
∼
east, at 80 arcsec from the X-ray peak. The feature is accompanied by a
forward shock in the same axis (NW-SE); moreover a surviving cooling core
is detected.
Despite the limited quality of the data, A2065 shows clear signatures of be-
ing an ongoing merger in the compact phase, when the detection of strong
shocks is the most favourable. This is similar to what is observed in numer-
ical simulations of head-on collisions of merging clusters. The fact that the
core of the main cluster is cool suggests that this is probably the remnant
of a cooling core, and thus the colliding object was probably of smaller mass
(e.g. G´omez et al., 2002). The new XMM-Newton observations of this object
should allow us to put better constraint on these preliminary results.
A3921
Observational merger evidence includes:
(1) TwopeaksinboththeX-rayemissionandthegalaxydistribution(Ferrari et al.,
2005);
(2) The hot bar is oriented parallel to the line joining the subclusters, it is
not orthogonal, as in the case of A1750;
(3) The central regions of the main cluster and the subcluster to the west
are strongly perturbed;
(4) There is an off-set between the galaxy distribution of the smaller sub-
cluster and the secondary X-ray peak (Belsole et al., 2005; Ferrari et al.,
2005).
This evidence cannot be interpreted as the result of a pre-merger. We are
seeing maybe the best example of an X-ray observed off-axis post-merger.
The subcluster has come from somewhere in the SE and is currently exiting
towards the NW. The two merging units have different masses, on a ratio of 1
to 3 (or 1 to 5; see also Ferrari et al. 2005). Comparison of these high quality
X-ray results with optical observations and numerical simulations yield an
estimate of the age of the merger of order 0.5 Gyr after core passage. Off-axis
mergers aremorelikely tooccur thanhead-onones,andthey aremoreefficient
in mixing the gas via turbulence. If anything, A3921 needs even deeper study,
and the combination of these XMM-Newton data with upcoming Chandra
observations will give further elements to our interpretation.
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5 Conclusions
The XMM-Newton observations of this small sample of galaxy clusters has
confirmed that the comparison of X-ray morphology and temperature is an
excellent tool to understand the dynamical status of these objects. In the
case of A1750 and A3921, spectroscopy of discrete regions ((Belsole et al.,
2004, 2005) has confirmed the significance of the temperature structure found
with the multi-scale wavelet approach. The significance of the temperature
structure in A2065 will be investigated with the upcoming XMM-Newton ob-
servation. For the better studied cases we have confirmed the previous inter-
pretation of a recent merger (A1750),but have added new evidence suggesting
that at least one of the subclusters is itself a merger remnant. Good quality
X-ray data have allowed us to completely revolutionise the interpretation of
the dynamical state of A3921.
We can organise these clusters along an evolutionary path, where A1750 rep-
resents the beginning of a merging event, which in a timescale of order 1.5
Gyr will be in the same state as A2065, when a bow shock is departing in the
direction of motion after the two cores have collided, and the two (or maybe
more) colliding objects are not physically separable anymore. Finally, A3921
represents the epoch when the secondary object has already passed the close
core passage phase, it is exiting on the far side with respect to the direction
of motion, and it will be accreted by the main cluster on a larger timescale
(of order 3-5 Gyr).
From so small a sample we cannot extract global conclusions on the popu-
lation of merging galaxy clusters. However, this work supports the necessity
of a wider investigation of the effect of the physics of mergers on the global
characteristics of galaxy clusters, especially if these objects are to be used to
derive cosmological parameters.
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